The long-term objective of this project is to determine the function(s) of dystrophin in striated muscle in order to understand how its absence or abnormality leads to the pathologies observed in Duchenne (DMD) and Becker (BMD) muscular dystrophies. During the most recent project period, we uniquely employed the baculovirus expression system and an array of biochemical/biophysical assays to define the actin binding properties of dystrophin and its homologue utrophin. We also demonstrated that dystrophin functions to organize the microtubule lattice of skeletal muscle through a direct binding interaction. Finally, we demonstrated that a baculovirus-expressed TAT-micro-utrophin construct shows promise as a novel protein replacement therapy for DMD. Based on these published results and exciting new preliminary data, we propose three new specific aims to: 1) test the hypothesis that disease-causing missense mutations in dystrophin cause a loss of function through protein aggregation, 2) determine the functional importance of microtubule lattice organization in skeletal muscle, and 3) reinvestigate functional necessity for the dystrophin carboxyl-terminal domain in skeletal muscle. The project will employ biochemical/biophysical methods to characterize a variety of dystrophin and utrophin protein constructs in combination with complementary experiments in isolated cells and transgenic mice. The proposed studies will address several fundamental questions about the function of dystrophin in normal skeletal muscle and how its absence or abnormality causes human diseases of skeletal muscle.